A number of applications presently exist for signalling lighting fixtures where the light output is required to be intermittent, or turned on and off. Perhaps the most widely used example is in traffic signal systems, but other examples exist, such as railroad signals, and other forms of flashing lights, "walk/don't walk" signals, and so forth. Intermittent signal lights of these types are almost invariably fitted with incandescent lamps and utilizes various types of switch gear to apply and remove the lamp voltage as desired. In this manner, the lamps are continuously turned on and off by the signalling system with which they are used.
Apart from home applications and purely decorative lighting, the proportion of lighting in the United States which is achieved using incandescent lamps is quite small. The reasons for this relate chiefly to the much higer efficiencies which may be obtained by the use of fluorescent and high-intensity discharge (HID) light sources. The continued use of incandescent lamps is not without merit, since the small light-producing area allows accurate optical control. In addition, incandescent lamps may be dimmed with ease and may be rapidly switched on and off without problems associated with fluorescent and HID light sources. It is this latter capability of incandescent lamps which has made them the widely used choice in intermittent signal light devices, such as traffic signals mentioned above.
Because of the relatively high power consumption of incandescent lamps, however, traffic light signals are becoming increasingly expensive to operate. This has been true, particularly in the most recent past, due to the significantly increased costs of electrical power. The power consumption of a traffic light no longer is negligible and constitutes a significant expense for municipalities operting even relatively small numbers of traffic signal lights.
With recent developments in lamp and electrical technology, the advantages of incandescent lamps for many appications are becoming eroded. HID light sources now may be obtained with approximately the same overall dimensions as incandescent lamps. Furthermore, the miniature HID lamps have light-producing areas roughly equivalent to the size of incandescent filaments. A high degree of optical control can be achieved using certain types of HID lamps, along with the very large gain in lumen output, because optical control possibilities are considerably greater when the light-producing area is small. For example, vehicular headlights traditionally have used incandescent lamps. In underground transportation, however, mine cars now are fitted with high-pressure sodium headlights. Great advantages are achieved, particularly in view of the very high power costs for operating incandescent lamps in mining situations. The small sizes now obtainable in HID lamps permit precise beam control of these lamps on mine cars, where a few years ago, the use of HID lamps in this environment was not practical. Although the initial costs of HID lamps are high, the savings in operation more than justify their use and, over a relatively short period of time, the lower costs of operation result in overall savings in spite of the high initial costs.
In order to employ fluorescent or HID lamps in a traffic signal, however, a means must be available to control and precisely time the on/off light for each signal face. At the present state of technology, it is unusual to switch HID or fluorescent lamps with the frequency required. Electronic devices are available to provide such switching, providing "instant restrike". These devices, however, are quite new and are relatively expensive. In addition, the effect of instant restrike devices upon lamp life and lamp efficiency and reliability is not known. To utilize electronic "instant restrike" control with HID or fluorescent lamps as a substitute for incandescent lamps in present traffic light fixtures, therfore does not appear to be a practical approach at the present time.
If HID lamps or fluorescent lamps can be continuously energized wtih a means for rapidly switching the passing or blocking of light from the lamp through the lens of a traffic signal, for example, significant energy savings in the operation of a traffic light may be accomplished. This results from the high efficiency of fluorescent lamps or HID lamps which produce large quantities of light with very low power consumption. Furthermore, more increased lamp life can be achieved, since fluorescent lamps can operate in excess of 24,000 hours.
Some attempts have been made in the past to provide a traffic signal light system using a continuously burning incandescent lamp. These systems, however, employ non-standard (by today's standards) traffic signal lamp housing to accomplish their purposes. Two U.S. patents which are typical of such systems are the patents to Paul, U.S. Pat. No. 2,136,804, and Heikes, U.S. Pat. No. 2,865,017. These patents disclose motor-driven rotating color discs for changing the color of the light passing through a single light-projecting aperture in each of the four directions controlled by the traffic signal. Both of these patents disclose the use of rotating discs mounted on horizontal axes in front of each of these apertures which move translucent colored panels in front of the apertures to produce the desired colored light projection from the respective apertures. The Paul patent also discloses a drum-type of rotating member mounted on a vertical axis for accomplishing the same purpose. Thus, at any given time, the two opposite apertures have green discs in front of them while the other two opposite apertures mounted at 90 degress to the first two, would have a red disc in front of them. The mechanisms of both of these patents require electric motors and a relatively complex gear arrangement to keep everything rotating and in synchronism.
Two other patents, both older than the Paul and Heikes patents mentioned above, disclose motor-driven vertical drums with different colored sections on them to produce the desired colored light through apertures on the four sides of the traffic signal device disclosed in these patents. Such patents are the U.S. patents to Wright, U.S. Pat. No. 1,640,170, and Johnson, U.S. Pat. No. 1,747,050. Both of these patents disclose the use of pawl and ratchet mechanisms for stepping the rotating drum from one position to another to sequentially place different colored lenses in position between the lamp and the apertures of the traffic light housing. Both of these patents require the use of a motor-driven mechanism to either rotate a cam or some other type of switch device to provide the pulses to the magnetically-operated pawl and ratchet mechanism. Consequently, they are fairly cumbersome and consume substantial amounts of power for the incandescent lamp, solenoid magnets, and clock or timing motors, all of which operate constantly.
Accordingly, it is desirable to develop a switched on and off signalling system which may be used in traffic signal light systems, or the like, which is capable of operation with fluorescent lamps or HID lamps (continuously energized) and which also has a reliable and simple means for attenuating the light passing from the lamp to the lens of such signal.